Coherent lightwave communication systems have grown in interest as the wavelength stability, wavelength tunability and manufacturable reproducibility of semiconductor lasers has been improved. In most coherent lightwave communication systems, optical heterodyning has been the suggested reception technique. In an optical heterodyne receiver, received lightwave signals and lightwave signals from a local oscillator are directed onto a photodetector. See S. D. Personick, Fiber Optics: Technology and Applications, pp. 242-245 (Plenum Press 1985). If the frequencies of the received and local oscillator lightwave signals differ, the resulting lightwave signals experience beating at a frequency corresponding to the difference in frequency of the received and local oscillator lightwave signals. Electronic (intermediate frequency) signals resulting from this interaction are directed to a square law mixer (for intensity or amplitude modulated signals) for further processing and detection. Beating causes oscillation in the photoresponse of the detector at the same difference frequency.
In a proposed optical heterodyne receiver, received lightwave signals are directed onto a semiconductor laser biased above threshold to interfere with lightwave signals being produced therein via optical mixing. See Sov. Phys.-JETP, Vol. 39, No. 2, pp. 522-7 (1974).
In such systems, whether real or proposed, the semiconductor laser has been operated primarily as dedicated receiver. As a result, another laser source is necessary to produce lightwave signals for full duplex of half duplex transmission on either communication system. Such a duplication is costly in that it doubles laser and electronic drive circuit costs plus it requires additional mounting and optics to handle separate reception and transmission.